Respiratory motion poses great challenges to radiotherapy of liver cancer. 4D-CT is the current clinical standard for imaging organ respiratory motion. However, it 1) involves high imaging dose to the patient due to prolonged scan time, and 2) provides insufficient soft tissue contrast for imaging liver tumor. Conversely, MRI has excellent soft-tissue contrast and has no known radiation hazard for clinical imaging. MRI-based 4D imaging (4D-MRI) is thus highly desired to improve the visualization of soft-tissue respiratory motion and the delineation of target volume of moving liver tumors. At present, there is no established 4D-MRI technique for radiotherapy. Our long-term goal is to establish a novel 4D-MRI based motion management framework for radiotherapy. The objective of this application is to develop and evaluate a clinical feasible 4D-MRI technique for improved imaging of liver tumor respiratory motion. Our overall hypothesis is that integration of 4D-MRI into radiotherapy of liver cancer will allow for more accurate delineation of tumor internal target volume (ITV). This hypothesis will be tested via two specific aims. Aim 1 will develop and validate a retrospective 4D-MRI technique that can be immediately implemented in the clinic. Aim 2 will evaluate accuracy of the tumor ITV determined from 4D-MRI and its dosimetric impact on treatment planning in liver cancer patient cases. Successful completion of these studies will provide the foundation required to establish a novel 4D-MRI based motion management framework for radiation therapy of liver cancer, and the necessary information to support a later larger scale study to further enhance the 4D-MRI tool. The fully developed 4D-MRI technology will improve our ability to more precisely deliver radiation treatment to tumor and mitigate radiation-induced injury to surrounding normal tissues.